Communication system



R. H. CAMPBELL COMMUNICATION SYSTEM June 13, 1939.

2 Sheets-Sheet 1 l I l 1 I l 1 l i Filed July 19, 1957 June 13, 1939. R. H. CAMPBELL commumcnlon SYSTEM Filed July '19, 1937 2 Sheets-Sheet 2 Patented June 13, 1939 The present Richard 1!. Campbell,

Webster Electric Company, poration of Wisconsin PATENT orrlca Racine, Wis winner to mine, Wia, a cor- Application July 19, 1937, Serial No. 154,344

23 Claims.

invention relates to communication systems and more particularly'to intercommunication systems of tral station and one Oneof the primary objects of the the type comprising a cenmore remote stations. Present invention is to provide a system with a simplified circuit arrangement normally conditioning the system for communication from one of the remote stations to the central station. but enabling an operator at the, central station to control the direction of communication and to communicate with any one of the remote stations to the exclusion of the remainder.

Intercornmunication systems of the type to which this invention relates, and comprising a central station and a plurality of remote stations each provided with suitable voice receiving and transmitting meansand an amplifier to the input and output of which they are selectively connected, should be available for instantaneous operation. That is, an operator .at either one of the remote stations, or at the desiring to communicate with the other central station not be required .to wait an appreciable length of time for the system to become operative. On the other hand, the system should not pick up and amplify local electrical disturbances.

It is, therefore, another object of the present invention to provide a communication system comprising an ampli ative and, therefore,

amplifying undesired which is rendered fier that is normally inoperincapable of picking up and electrical disturbances, but operative whenever control means at one of the remote stations is operated the central station or more, of

to condition the system for communication with or whenever control means at the central station is operated to condition the system for communication the remote stations.

The present invention is an improvement over therefrom to any one,

the invention disclosed and claimed in my copending application Serial No. 150,951, filed June 29, 1937, entitled Communication apparatus and assigned to the assignee of This copending application cation.

the present appliis directed more particularly tothe arrangement for selecting the stations with which it is desired to communicate and to the provision of control switches at both the central and remote stations.

inventionwill becom description in the 0 Further objects and advantages of the present e apparent fromthe ensuing curse of which reference is had to the accompanying drawings, in which;

Fig. 1 is a schematic diagram of one embodimodification of the in Fig. l; and

ment of my invention;v

Fig. 2 is a schematic diagram illustrating a circuit arrangement shown Fig. 3 is a similar view illustrating still an- 'co; other modification o f the invention.

should the type employing a permanent magnet and a voice coil suspended in the field thereof, located at a station from which the communication system is to be controlled. This station may be aptly termed either a master or central station, and hereinafter will be referred to as a central station.

Loudspeaker units ll similar in all respects to unit Ill are located at one or more remote stations between which and the control station it is desired to communicate.

, The fluctuations in. current produced in speaker units when acting as transmitters are amplifled by an amplifier II of a construction that is to" be described in detail shortly. The amplifier comprises the usual input transformer having a primary. coil. l3 and an output transformer having a secondary coila-l'l, each of which has one end thereof connected to a common conductor l5 which is suitably grounded as at It.

A selectively operable master switch I! is provided at the central station for connecting the speaker unit Ill thereat to either the output or input of the amplifier, and at the same time either condition the remote speaker units II for connection to the input of the amplifier through normally open push button switches 18 at the remote stations or directly connect them to the amplifier. Switch I1 is biased by suitable means, such as aspring, to a normal position in which the speaker unit I0 is connected to the output, and units ll are conditioned for connection to the input of the amplifier by their respective switches I8, whereby any of the remote stations may communicate with the central station.

The switch ii is a double pole double throw switch connected for operation as a reversing switch and, in order to provide a system using a minimum number of conductors, the switch blades 2i and E2 thereof are connected by con-.

ductors 23 and 2G to the speaker units H and Ill, respectively. The input coil iii is connected by a conductor 25 to the interconnected contacts 26; and 21 and the output coil is connected by conductor 28 to the interconnected contacts 29 and 30 of the reversing switch.

In the normal position of switch it the voice coil associated with speaker unit ill is connected across the output of the amplifier through conductors 24-, 28 and switch blade 22 in engagement with contact 30 and a direct connection through common conductor ii. In the displaced position of the switch the coil is connected across the input through conductors 24, 25 and switch blade 22 in engagement with contact 21.

In the normal position of switch 11 the voice coils associated with remote speaker units H are conditioned for connection to the input of the amplifier by the push button switches l8 at the various remote stations through conductors 23 and 25 and switch blade 2| in engagement with contact 26. When closed, the switches l8 complete the circuit to the ground conductor |5 through an extension l5 of the latter and a plural position selector switch 3| at the central station normally occupying a position in which it connects conductors I5 and IS.

The plural position selector switch 3| is provided to enable the operator at the central station to communicate with any one or all of the remote stations. When the operator desires to communicate with one station, he can do so and exclude all others from the conversation.

In the illustrated embodiment of the invention the three remote stations H are connected by conductors 32, 33, and 34, to separate contacts adapted selectively to be engaged by the switch 3|. Thus, whenever the switch 3| is operated to connect a circuit from the common conductor l5 to any of the three last mentioned conductors, one of the remote stations H is connected to the common conductor 5, and the station selected is placed in communication with the central station. At this time the direction of communicationis controlled solely by the operator at the central station, and further operation of the switch |8 does not aiTect the system in any way, because conductor I5 is no longer connected to the common conductor 5. When the operator at the central station desires to talk, he places switch I! in its displaced position, thereby connecting speaker ID to the input and the selected speaker II to the output. The connection of speaker l0 has been traced previously and the connection to the selected speaker H extends from the output coil |4 through conductor 28, switch 2| in engagement with contact 29, conductor 23, to the voice coil of the speaker and thence back to the output through one of conductors 32, 33 or 34, switch 3| in contact therewith, and conductor l5. When the operator at the central station desires to listen, he simply releases switch I! which then returns to its normal position in which the speaker I0 is connected to the o'utput coil and the selected speaker H to the input coil, through the circuits described above.

To enable the central station to communicate simultaneously with all remote stations, the selector switch is operable toanother position in which it engages a group of contacts 35 each connected to a voice coil of one of the loudspeaker units I. When the selector switch 3| is operated to this last position, all of the remote stations are connected directly to the amplifier so that conversation may be carried on between any one or all of the remote stations and the central station under the control of the operator at the central station.

When the amplifier 2 of the communication system is conditioned for instantaneous operation with both the cathode and anode circuits energized and the speaker units connected to the terminals of the amplifier, there is a possibility that local disturbances will be impressed upon the input of the amplifier and consequently create undesired noise at the central station. In the present embodiment, this undesired operation of the system is avoided by normally disconnecting the anode circuit of the amplifier so that the amplifier is'normally inoperative. However, in order to avoid any delay before communication can be established, the cathodes of the electron discharge devices of the amplifier are maintained at operating temperatures at all times.

The amplifier control is also so arranged that the amplifier may be placed in operative condition by the operators at the various'remote stations or by the operator at the central station.

This control is achieved by means of a normally open switch 38 at each remote station and a switch 40 at the central station. The switches 38 are mechanically connected with the push button switches l8 and are, therefore, operable simultaneously with the latter, andthe switch 40 is similarly arranged with respect to the selector switch 3|. Before describing the details of the amplifier circuit and its power supply, it may be well to note that when the system is in its normally condition, the normally open anode circuit of the amplifier may be completed through the switches 38 by operators at the remote sta tions and through the power control switch 40 by the operator at the central station.

While the principles of the present invention may be embodied in communication :-ystems utilizing various types of amplifiers, the amplifier 12 of Fig. 1 has been illustrated as comprising a pair of vacuum tubes 4| and 42. The control electrode of vacuum tube 4| is connected by a conductor 43 to one terminal of the secondary coil 44 of the input transformer, the other ter minal of the coil being connected to common conductor l5 through a resistor 45. The cathode of the tube is connected to ground through the cathode biasing resistor 46 to which the suppressor grid is connected by a conductor 4? in accordance with well known principles. A bypass condenser 43 is placed in a circuit connecting the junction of coil 44 and resistor to the junction of the suppressor grid and cathode. The anode of tube 4| is connected to the control grid of tube 42 through a conductor 49 having interposed therein a blocking condenser 50.

Suitable potentials are impressed upon the anode and screen grid through the plate resistor 5| and screen grid resistor 52, respectively. These potentials are obtained from a suitable power source to be described shortly through a conductor 53, leading to the positive B terminal.

The values of the potentials impressed upon the plate and screen grid are determined further by a resistor 54 connecting the screen grid to conductor I5 and forming part of the direct current circuit ofthe power supply. A screen grid by pass condenser 55 is connected in shunt with the resistor 54,

The screen and plate of the tube 42 are supplied with potentials from the positive B terminal by a conductor 56, connected to the screen through a branch conductor 51, and to the plate through the output transformer primary coil 58, associated with the output coil l4. The cathode is connected directly to the common conductor l5 by a conductor 59 and the suppressor grid is connected to the cathode by a conductor '60. The control electrode of tube 42 is suitably biased through a grid biasing resistor 6| connected through conductor 62 to potentiometer resistance 63, connected in turn across the terminals of the choke coil 64 forming part of a rectifier. Conductor 62 is connected to conductor I5 through a by-pass condenser 65.

The power is supplied to the amplifier from any suitable alterating current source through the transformer 66, having associated therewith a secondary winding 61 connected in circuit with the various cathode heaters and a secondary conductor l5.

ance with the usual practice the midpoint of the secondary winding 68 is connected by a conductor II to a filter comprising the choke coil 64 and a pair of condensers l2 connected across the conductor 1i and a conductor 13 leading from the cathode of the rectifier tube to the positive 13 terminal. The end of the choke coil remote from the rectifier is connected by a conductor 14 to the negative B terminal.

The cathodes are normally crating temperature so condition for instantaneous operation whenever a circuit is established from the source of power to the anodes of the tubes. Hence, since the positive B terminal is at the screen grids and plates of tubes 4| and 42, the amplifier is rendered operative whenever the negative B terminal is connected to the grounded Asstated above, this connection may be made by the operator at the central station through switch M and operators at the remote stations through switches 38.

When the system is in its normal condition, the operators at the remote stations can render the amplifier operative by closing switches 33, thereby connecting the negative B terminal to ground through conductor 15, switch 40 in its normal position, a conductor 16 connected in parallel to one contact of each of the switches 38, the other contacts of which are connected to conductor l5 heated to their opthrough conductor l5 and switch 3|.

" eration of switch When the system is in normal condition the operator at the central station is able to place the amplifier in operativecondition by actuation of selector switch 3| to establish a circuit between conductor l5 and any of the conductors 32 to 34, inclusive, or into engagement with contacts 35. This operation of switch 3| effects op- 40 into engagement with an arcuate contact bar 18 connected by a conductor 19 to a common conductor l5. Whenever the switch 40 engages the contact bar 18 the negative B terminal is connected to the ground directly through the switch 40, contact bar 18 and conductor 19.

In operation the transformer 66 is connected to a suitable source of alternating current through a switch (not shown) providing a means for controlling the energization of the entire system. Immediately upon the connection of the primary winding of the transformer to the source of power, the cathode heaters are supplied with energy through the secondary winding 6'! and the secondary winding 68 is also energized. Under normal conditions the rectifier is not operative because the direct current. circuit therefor is broken at the switches 38. This direct current circuit extends from the cathode of the rectifier tube through conductor 13 to the positive B terminal and from there through conductor 53, resistors 52and 54 in series to the ground conductor id. The ground return i extends from the common conductor IE to the conductor 15 either through the conductor l5 leading to the remote station, or the branch conductor!!! leading to the contact bar 18.

Under the normal conditions presently being considered, the ground return is completed through switches 38 that the amplifier is in all times connected to.

tifier through the conductor stations.

station, he merely closes the switch l8-38 thereby completing the connection of the speaker unit II at the remote station to the input of the amplifier, and placing the amplifier in operation. The amplifier is operative immediately and no delay in establishment of communication occurs.

If the operator at the central station desires to communicate with only that remote station calling *him, he operates a switch 3| to establish a circuit with that station through one of the conductors 32, 33, or 34. Upon this actuation of switch 3|, the switch M) is simultaneously operated to establish a ground return to the rec ,19 and contact bar 18, thereby maintaining the amplifier in operative condition as well as connecting the loudspeaker at the selected remote station directly to the ground conductor l5. 'Thereafter the central station and remote station may communicate with each other, the direction of communication being under the full control of the operator at the central station who operates the master switch H to its displaced position to talk V to the remote station and allows the master switch to return to its normal position whenever he desires the operator at the remote station simultaneously to communicate with all stations he operates the selectorswitch 3| into engager ment with contacts 35, thereby connecting all the speakers II in parallel. Simultaneously with this operation the switch 40 is operated to establish the previously described ground return to render the amplifier operative.

The modified form of the invention illustrated in Fig. 2 differs from the modification of Fig. 1 in that the completion of the normally open ground return circuit from the rectifier to the amplifier is controlled by means of a relay adapted to be energized from either the central or remote stations. The relay, indicated generally by reference numeral 80, is adapted to be energized from an alternating current source of power, preferably the cathode heater transformer winding 61. One terminal'oi the windingis connected by a conductor 8| directly to the relay winding and the other terminal is connected by a' conductor 32 to the contact bar i8- and by a branch conductor 83 to one terminal of each of the switches 38 'at the various remote The other terminals of the switches 38 are connected by a conductor 84 to a contact adapted to be engaged by the control switch 40 in its normal position. switch M] is connected by'a conductor 85 to the other terminal of the relay windingl The relay has associated therewith. 2. normally open switch 86 adapted to close. a directly across conductors l5 and I9 tocomplete the ground return whenever the relay 80 is energized. v a

The operation of the communicationsystem illustrated in Fig. 2 is substantially the same as that of the previously described modification. Thus, whenany of the operators at the remote circuit stations desire to communicate with the central station they simply operate the push button switches |838 at the remote station to their closed positions. With the switches 3| and 40 in their normal positions, this operation of the switches i8-38 results in the connection of the remote speaker to the input and the energization of the relay 8G to complete the ground return for the rectifier.

The energizing circuit for the relay extends from one terminal of the winding 61 to the relay through conductors 82, 83, switch 38 in its closed position, conductor 84, switch 40 in its normal position, and conductor 85, and from the other terminal of the relay 80 to the secondary winding through conductor 8|.

Whenever the operator at the central station desires to communicate with a selected one or all of the remote stations, he simply operates switch 3| to the desired position. Upon such operation of the switch 3|, the switch 40 engages the contact bar 18, thereby connecting the relay winding directly across the terminal of the secondary winding 61 through conductors 8|, 82 and 85.

The modification of the invention illustrated in Fig. 3 differs from the previously described modifications in that the grid bias of one of the tubes of the amplifier is governed to control the operative condition of the amplifier.

Under normal conditions the amplifier is rendered inoperative by connecting the screen grid of tube 4| to ground, thereby rendering the tube 4| non-conductive. Whenever the amplifier is to be used the aforementioned connection is broken and the proper potential applied to the screen grid to render tube 4| conductive and the amplifier operative. This arrangement, like the previously described arrangements, conditions the amplifier and communication system for immediate transmission of speech in either direction.

Referring now specifically to Fig. 3, the system illustrated therein comprises an amplifier l2 having its cathode heaters and anodes energized in such manner as to render the amplifier ready for immediate operation whenever the proper potential is applied upon the screen grid of tube 4| as described hereinafter. For this reason the negative B terminal is connected directly to the common conductor I5 through conductor 90.

The amplifier circuit connections are substantially the same as those of the previously described modifications, the only difierence residing in the provision made normally to connect the screen grid directly to ground by short circuiting resistor 54.

Inasmuch as the positive B terminal is connected to the plates of tubes 4| and 42 through circuits including conductors 53 and 56, it is evident that the rectifier is operative and its output impressed across the plates and grids of the tubes. The rectifier circuit extends from the positive B terminal through conductor 53, resistor 52 directly to the negative B terminal through the short circuit around resistor 54 to be described shortly. Since the negative B terminal is grounded, the screen grid of tube 4| is normally at ground potential to render the tube non-conductive, but this does not materially afiect the operation of the rectifier because the screen grid resistor 52 is of a high value (in the neighborhood of 250,000 ohms) in accordance with usual practice.

The resistor 54 is normally short circuited through a circuit including the selector switch 3| at the central station, and series connected normally closed switches 9|, 92 and 93 at each of the remote stations and operable simultaneously with switches 18 thereat. One terminal of the resistor 54 is connected directly to common conductor l5, as in the previous modifications, but the other terminal has an additional connection through conductor 94 to one contact of switch 9|. The other contact of switch Si is connected by conductor 95 to one contact of switch 92, the other contact of which is connected to one contact of switch 93. The remaining contact of the latter is connected directly to conductor I5 to complete the series connection of the switches.

The operation of the system as a whole is substantially the same as that of the previously described modifications, and consequently it is deemed necessary only to describe the operation whereby the amplifier is rendered operative under either the control of the operator at the central station, or the operator at any one of the remote stations.

With the transformer 66 connected to a suit able source of power and the various switches in their normal and illustrated positions, any one of the operators at the remote stations is able to establish communication with the central station and render the amplifier operative by operation of. switches thereat to their displaced positions. Thus when the operator at the first station operates switch |8--9| to its displaced position, the speaker unit ll thereat is connected by switch IE to the input of the amplifier, and the switch 9| breaks the short circuit across resistor 54. Accordingly, the proper operating potential is impressed upon the screen grid and the amplifier rendered operative.

When the operator at the central station responds to a call from the first station, he operates switch 3| to establish a circuit from conductor l5 to conductor 32, thereby rendering the circuit including the switch |89| inefiective and likewise removing the short circuit across resistor 54 which, as described above, comprises switch 3| in its normal position. The operator at the central station controls the direction of communication at all times, and when a conversation is. concluded he returns switch 3| to its normal position, thereby again conditioning the system for a call from any one of the remote stations upon operation of the switches thereat.

Thus, as in the previous modifications, either the operator at the central station, or any one of the remote stations is able to establish communication immediately upon operation of a single switch at his station.

While only three remote stations have been illustrated in the description of the invention, it is apparent that the number of stations can be altered at will merely by modifying the number of contacts associated with switch 3| and, in the case of the two first described modifications, the length of the contact bar 18. Furthermore, the invention is applicable to a system using anamplifier having more than two stages of amplification and it is not necessary that the grid potential of the tube in the first stage be controlled to control the operative condition thereof.

What I claim as new and desire to secure by United States Letters Patent is:

l. A two-station intercommunicating system, including in combination, voice receiving and transmitting means at each station, an amplifier,

.ditioning the: voice means including a reversing switch at one station for selectively connecting the voice receiving and transmitting means thereat directly to the output and input of. the amplifier and conreceiving and transmitting means at the other station for connection to the input and output of the amplifier, and control means at the second station for completing the connection of the voice. receiving and transmitting means at said station to the amplifier.

2. A two-station intercommunicating system, including in combination, a loudspeaker unit or the type adapted to function as a microphone and loudspeaker and comprising a voice coil at each station, an amplifier of the type having one terminal of its input and output connected to-a common conductor, means including a reversing switch at a first station and a single conductor leading therefrom to one terminal of the voice coil associated with each speaker for alternatelyconnecting the said terminals of said coils di-. rectly to the other terminals of the input and output of the amplifier, a direct connection from the other terminal of the voice coil at the first station to the common conductor whereby'the connection of the speaker unit at the first station to the amplifier is controlled by said reversing switch, and a circuit including a normally open switch at the second station for connecting the other terminal of voice coil of the speaker thereat to the common conductor.

3. A two-station intercommunicating system, including in combination, a loudspeaker unit of the type adapted to'function as a microphone and loudspeaker and comprising a voice coil at each station, an amplifier having one terminal of its input and output connected to a common conductor, means including a reversing switch at a first station and a single conductor leading therefrom to one terminal of the voice coil associated with each speaker for alternately connecting said terminals of the coils directly toother terminals oi the input and output, said reversing switch being biased to connect the voice what the first station to the output and that at the second station to the input, a direct connection from the other first station to the common conductor and a normally open switch for connecting the other terminal of the voice coil at the second station to the common conductor, whereby the system is normally conditioned for communication from the second station to the first upon closure of the normally open switch.

4. An intercommunicating system, including in combination, a central station, a plurality of remote stations, voice receiving and transmitting means at each station, an amplifier, means including a reversing switch at the central station for selectively connecting the voice receiving and transmitting means thereat directly to the output and input of the amplifier and simultaneous- 1y conditioning the voice receiving and transmitting means at the remote stations for connection to the input and output of the amplifier, and control means at'each remote station for completing the connection of the voice receiving and transmitting means thereat to the amplifier.

5. An intercommunicating system, including in combination, a central station, a plurality of remote stations, voice receiving and transmitting means adapted to function as microphones and loudspeakers at each station, an amplifier, means including a reversing switch at the central station for selectively connecting the voice receiving terminal of the voice coil at the and transmitting means thereat directly to the input and output and simultaneously partly con-' and transmittingditioning the voice receiving means at the remote stations for connection to the output and input of the amplifier, means mcluding a selector switch at the central station .for selectively completing conditioning of the voice receiving and transmitting means at the remote stations to the amplifier, and control means at each remote station for completing the connection of the voice receiving and transmitting means thereat to the amplifier.

6. An intercommunicating system, including in combination, a central station, a plurality of remote stations, speaker units adapted to function as microphones and amplifiers at each station, an

amplifier having one of its input and output terminals connected to a common conductonmeans including a reversing switch connected to the other terminals of the input andoutput, a first conductor leading therefrom to the speaker unit vat the central station and a second conductor leading therefrom to all the remote speakers for selectively connecting the conductors to the output and input, said switchbeing" biased to a normal position in which the first conductor is connected to the output and the second conductor to the input, a direct connection from the speaker at the central station to the common conductor, said common conductor having an extension to the remote stations that is adapted to be connected to the previously mentioned portion thereof, a plural position selector switch at the central station adapted in one position to connect the extensionto the common conductor, a branch circuit comprising a normally open switch at each remote station for completing the connection of the speakers thereat to the input of the amplifier when the reversing switch is in its normal position and one position, from speakers at each remote station to the plural position selector switch whereby the latter may be selectively operated directly to connect the speakers at the remote stations to the amplifier and the communication to the selected station controlled solely by the reversing switch.

7. A multiple station intercommunicating system, including in combination, (mice receiving and transmitting means adapted to function as microphones and-loudspeakers at each station, amplifier means normally in aninoperative condition, lectively connecting the voice receiving and transmitting means thereat to the output and input of the amplifier means and for conditioning the voice receiving and transmitting means at a second station for connection to the input and output of the amplifier, and control means at the second station for simultaneously completing the connection of the voice receiving and transmitting means thereat to the amplifier and rendering said amplifier means operative. 8. A multiplestation intercommunicating system, including in combination, voice receiving and transmitting means adaptedito function as of the amplifier and for partly conditioning the voice receiving. and transmitting means at a second station for connection to the input and out-- put of the amplifier, and means including a sethe selector switch is in its said and branch circuits leading directly control means at a' first station for selector switch at the first station adapted selectively to complete the connection of the voice receiving and transmitting means at the remote stations for rendering the amplifier 'operative upon such connection.

9. An intercommunicating system, including in combination, a central station, a plurality of remote stations, voice receiving and transmitting means adapted to function as microphones and loudspeakers at each station, amplifier means normally in an inoperative condition, means including a control switch at the central station for selectively connecting the voice receiving and transmitting means thereat to the output and input of the amplifier means and for partly conditioning the voice receiving and transmitting means at the remote stations for connection to the input and output of the amplifier, means including a selector at the central station selectively operable to complete the connection of any one or all of the voice receiving and transmitting means at the remote stations to the amplifier, and additional switch means operable simultaneously with the selector switch when the latter is operated to select any one of the stations for rendering said amplifier means operative.

10. An intercommunicating system, including in combination, a central station, a plurality oi. remote stations, loudspeakers adapted to function as microphones and loudspeakers at each station, amplifier means including one or more electron discharge devices normally in an inoperative condition, control means at the central station for selectively connecting the loudspeaker thereat to the output and input of the amplifier means and for simultaneously partly conditioning the loudspeakers at the remote stations for connection to the input and output of the amplifier, said control means being biased to a normal position in which the loudspeaker at the central stations is connected to the output and the loudspeakers at the remote stations partly conditioned for connection to the input, a plural'position selector at the central station adapted in a first position to complete the conditioning of the loudspeakers at the remote stations for connection to the amplifier and in the remaining positions to complete the connection of these loudspeakers to the amplifier, control means at each remote station for completing the connection of the loudspeakers thereat to the amplifier when the selector is in its first position, means efiective upon operation of the selector to any one of its said remaining positions for rendering said amplifier operative, and means operable simultaneously with each of said remotestation control means for rendering said amplifier operative when the selector is in its first position.

11. A multiple station intercommunicating system, including in combination, a central station, one or more remote stations, a loudspeaker adapted to function as a microphone and loudspeaker at each station, an amplifier including electronic devices comprising cathodes normally in condition for instantaneous operation and anodes normally disconnected from a source of potential whereby said amplifier is normally inoperative, control means at the central station for selectively connecting the loudspeaker thereat to the output and input of the amplifier and conditioning the loudspeakers at the remote stations for connection to the input and output of the amplifier, and control means at the remote stations for slmultaneously completing the connection of the speakers thereat to the amplifier and connecting the anodes to the source of potential to render said-amplifier operative.

12. A multiple station intercommunicating system, including in combination, a central station. one or more remote stations, a loudspeaker adapted to function as a microphone and loudspeaker at each station, an amplifier including electronic devices comprising cathodes normally in condition for instantaneous operation and anodes normally disconnected from a source of potential whereby said amplifier is normally inoperative, control means at the central station for selectively connecting the loudspeaker thereat to the output and input of the amplifier and conditioning the loudspeakers at the remote stations for connection to the input and outputof the amplifier, and additional control means at the central station for simultaneously completing the connection of any or all of the remote stations to the amplifier and connecting the anodes to the source of potential to render said amplifier operative.

13. A multiple station intercomr'nunicating system, including in combination, a central station, one or more remote stations, a loudspeaker adapted to function as a microphone and loudspeaker at each station, an amplifier including electronic devices comprising cathodes and anodes, a source of potential, a transformer comprising a primary winding, a secondary winding for heating said cathodes to render the amplifier ready for substantially instantaneous operation, means including a second secondary winding and a rectifier normally disconnected from but adapted to be connected to said anodes to render the amplifier operative, control means at the central station for selectively connecting the loudspeaker thereat to the output andinput of the amplifier and conditioning the loudspeakers at the remote stations for connection to the input and output of the amplifier, a switch at each remote station for completing the connection of the speaker thereat to the amplifier, and means including a switch operable simultaneously with the last mentioned switch and a relay connected thereby across the terminals of the first mentioned secondary winding for connecting the rectifier to the anodes, whereby said amplifier is rendered operative upon closure of the switches at the remote stations.

14. A multiple station intercommunicating system, including in combination, a central station, one or more remote stations, a loudspeaker adapted to function as a microphone and loudspeaker at each station, an amplifier including an electronic device comprising a cathode, an anode and a control electrode, a source of potential for said amplifier, means including a normally closed circuit for controlling the potential impressed upon the control electrode of said device for normally rendering said device inoperative, control means at the central station for selectively connecting the loudspeaker thereat to the output and input of the amplifier and conditioning the loudspeakers at the remote stations for connection to the input and output of the amplifier, and control means at each remote station for simultaneously completing the connection of the speaker thereat to the amplifier and breaking said normally closed circuit forrendering said amplifier operative.

15. A multiple station intercommunicating system, including in combination, a central station, one or remote stations, a loudspeaker adapted to function as a microphone and loudspeaker at each station, an amplifier including electronic devices comprising cathodes, anodes and control control electrode of one of said devices for normally rendering said device inoperative, control means at the central station for selectively connecting the loudspeaker thereat to the output and input of the amplifier and conditioning the loudspeakers at the remote stations for connection to the input and output of the amplifier, and additional control means'at the central station for simultaneously completing the connection of any of the remote stations to the amplifier and breaking said normally closed circuit for rendering said amplifier operative.

16. A multiple station intercommunicating system, including in combination, a central station, one or more remote stations, a loudspeaker adapted to function as a microphone and loudspeaker at each station, an amplifier including an electronic device comprising a cathode, an

anode and a control electrode, a source of potential for said amplifier, a common grounded conductor, a plural position selector switch at the central station, means including said selector switch in its normal position and series connected switches at the remote stations for normally connecting said control electrode to ground to render said'device nonconductive, branch conductors leading to the selector switch from the loudspeakers at the remote stations and adapted to be engaged thereby in its positions other than said normal position for connecting the loudspeakers to the amplifier, whereby said short circuit is broken to render said device conductive upon operation of said selector switch from its normal position, means including other branch conductors and normally open switches at each remote station for connecting the loudspeakers thereat 'to the amplifier through said selector switch in its normal position, each of said last mentioned switches being operable simultaneously with one of said series connected switches,

whereby said short circuit is broken to render said device conductive upon operation of said last mentioned switches to connect the remote loudspeakers to the amplifier.

17. An intercommunicating system, including in combination two or more remotely located stations, voice receiving and transmitting means of the type adapted to function as a microphone and'loudspeaker at each station, an 'am plifier, means including a reversing switch at one stationior selectively connecting the voice receiving and transmitting means thereat directly to the output and input of the amplifier and conditioning the voice receiving and transmitting means at the other stations for connection to the input and output of the amplifier, and control means at the said other stations for completing the connection of the voice receiving and transmitting means at said stations to the amplifier.

18. An intercommunicating system, including in combination two or more remotely located stations, voice receiving and transmitting means at each station, an amplifier, a communication control switch at one station for controlling the connection of the voice receiving and transmitting means to the amplifier, switch means at said one station selectively operable from a predetermined position to complete the connection of the voice each of said other stations system, the combination receiving and transmitting means at the other stations to the amplifier, and control means at connection of the voice receiving and transmitting means thereat to the amplifier through said switch means when terminedposition. I

19. In a multiple station intercommunicating system, the combination including normally inoperative amplifier means, a plurality of interstaticn conductors, means for selectively connecting said interstation conductors to said amplifier, and means operable simultaneously with said lastmentioned means for rendering operative said amplifying means. a

20. In a multiple station intercommunicating including normally inoperative amplifier means, a plurality of interstation conductors, selector means at one stathe latter is in said predetion for selectively connecting interstation conductors to said amplifier, said selector means being operable to and normally occupying a position in which said conductors are disconnected from said amplifier, means at another station for connecting that station to the amplifier through said selector means, and means operable simultaneously with said last-mentioned means for rendering operative the amplifier means.

21. In a multiple station intercommunicating system comprising a central station and a plu-' railty of remote stations, the combination including normally inoperative amplifier means at the central station, voice receiving and transmitting means at the central station normally connected to the amplifier, voice receiving and transmitting means at each of the remote stations, and means including a switch at each remote station for simultaneously connecting the voice receiving and transmitting means thereat to the amplifier means and rendering the amplifier means operative.

22. In an intercommunicating system the combination including an amplifier, a source of power for said amplifier, a voice reproducer located remote from the amplifier and adapted to be connected to said amplifier, a switch near said reproducer for connecting it to the amplifier, and other switch means operable simultaneously with said first mentioned switch for connecting said amplifier to said source of power.

23. In a multiple station intercommunicating system of the loudspeaking type, the combination including, an amplifier including electronic devices comprising cathodes and anodes, a source of potential, a transformer comprising a primary winding adapted to be energized from said source and a secondary winding for heating said cathodes to render said amplifier ready for substantially instantaneous a second secondary winding and a rectifier nor-- mally disconnected from but adapted to be connected in circuit to said anodes to render the amplifier operative, a plurality of interstation conductors, means for selectively connecting said interstation conductors to said amplifier, and means including a switch operable simultaneously with said last-mentioned means for complating the circuit from said rectifier to said anodesfor rendering the amplifier operative.

RICHARD H. CAMPBELL.

for completing the operation, means including DISCLAIMER 2,162,369.-R'ichard H. Campbell, Racine, Wis. COMMUNICATION SYSTEM. Patent dated June 13, 1939. Disclaimer filed November 27, 1944, by the assignee, IVebster Electric Company.

Hereby enters this disclaimer to claim 22 of said patent. v [Qfl'icial'Gazefle December 19, 1.944.] 

